Honestly I don't think it's anything to do with vaping, and everything to do with not smoking. Smoking is just that bad for you, it seems to affect how your body can fight off infection to some degree, and the longer you smoke the worse it gets.. In the last few years I was smoking, I was getting multiple colds and illnesses each year, and at least once a year was having a cold that I couldn't get rid of turn into pneumonia..
It might have to do with circulation that smoking interferes with, I don't know but not getting sick all the time anymore... is directly related to quitting smoking, and even though we vape, are experiencing the same benefits of not smoking that someone who was able to quit cold turkey experiences..
It's simply how smoking affects you, verses how not smoking affects you. There is no scientific data on this, it's simply my opinion.